In a typical known image forming apparatus, an image is formed by transferring a developer (toner) onto a recording medium, such as a recording sheet. An image forming apparatus of this type includes a conveyor belt for conveying the recording medium to an image transfer position.
In some cases, unintended and unnecessary developer adheres to the conveyor belt due to, for example, a sheet jam. In other cases where color printing is performed with a plurality of process cartridges arranged along a conveyance direction of the recording medium, a developer, which is used to form a correction pattern image for adjusting each image forming position of each of the process cartridges (i.e., registration control) and for adjusting a concentration of each of the process cartridges, sometimes adheres to the conveyor belt.
Since the developer not to be used to form a transfer image leads to deterioration of an image quality of a transfer image, the image forming apparatus usually includes a cleaning unit for cleaning the developer adhering to the conveyor belt.
In a typical cleaning unit, a cleaning roller is disposed at a position facing a conveyor belt. A voltage is applied to the cleaning roller to cause a potential difference between the conveyor belt and the cleaning roller, thereby transferring an electrically charged developer on the conveyor belt to the cleaning roller by an electrostatic force. Thus, cleaning (hereinafter also referred to as “bias cleaning”) of the conveyor belt is performed.
In this case, feedback control of the voltage applied to the cleaning roller is also performed in order to avoid differences in cleaning performance of the cleaning roller due to changes in using conditions or secular deterioration of the cleaning roller.
It is known that a status of the cleaning unit deteriorates depending on a number of times of use. Specifically, a load resistance between the conveyor belt and the cleaning roller is changed (usually increased) due to, for example, adherence of paper dust to fine pores formed in a surface of the cleaning roller.
However, there has been a problem that deterioration of the status of the cleaning unit due to use (and thus an increase in the load resistance) cannot be accurately specified since the status of the cleaning unit is indirectly specified based on, for example, a number of printed sheets counted by the cleaning unit.
For example, when the cleaning unit is replaced with not a new one but a used one or the like, the status of the cleaning unit indirectly specified based on the number of printed sheets is completely different from an actual status of the cleaning unit. Thus, cleaning is likely to be performed inappropriately.
In view of the possibility that a current flow exceeding a predetermined maximum value may cause damage of the conveyor belt, it is preferable to impose a current limitation. In the cleaning unit, in which voltage control is performed as described above, a separate component for current detection is needed, which leads to an increased size of an image forming apparatus.